glz::generic: name key and types in access errors

[stephenberry]

Closes #3001

glz::generic access errors now say what went wrong:

• Missing key on a const generic: glz::generic key not found: "missing" (still std::runtime_error).
• get<T>() type mismatch: glz::generic get: expected double, but holds string.
• Key or index on the wrong type: glz::generic key "key": expected object, but holds array / glz::generic index: expected array, but holds string.

Type mismatches throw the new glz::bad_generic_access, which derives from std::bad_variant_access, so existing handlers still catch it. Builds without exceptions still abort. The get_*() and as<T>() accessors get the new messages through get<T>().

Tests are in exceptions_test, and docs/generic-json.md is updated.

[packit-as-a-service]

One of the tests failed for b48c995. @admin check logs https://download.copr.fedorainfracloud.org/results/packit/stephenberry-glaze-3007/fedora-rawhide-ppc64le/11155420-glaze/builder-live.log, packit dashboard https://dashboard.packit.dev/jobs/copr/4087150 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/11155420/

[packit-as-a-service]

One of the tests failed for e9b81de. @admin check logs https://download.copr.fedorainfracloud.org/results/packit/stephenberry-glaze-3007/fedora-rawhide-ppc64le/11155939-glaze/builder-live.log, packit dashboard https://dashboard.packit.dev/jobs/copr/4087583 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/11155939/